Niobium (Nb), melting temperature 2450.degree. C., (also called Columbium) is a difficulty purified metal.
Pure metal applications of niobium are based on its properties of high corrosion resistance, high electrical conductivity, good ductility and its superconductivity below 9.2K.
There has been significant and growing utilization of rf superconductivity for nuclear physics accelerators and high energy accelerators. It is anticipated that the e.sup.+ e.sup.- storage rings LEP, TRISTAN, and PETRA will install large sections of niobium superconducting accelerating cavities, Tigner, 1983, IEEE TRANS., NS-30, 3309. Improvements in currently achievable performance levels (which are far below theoretical expectations) will lead to significant enhancements of these planned applications as well as to new applications. Currenty one of the dominant limitation mechanisms is "quenching" of the superconductivity near isolated regions of high losses--referred to as defects. Improved understanding of this phenomena shows that it can be considerably ameliorated by improving the thermal conductivity of the Niobium between 4.2 and 9.2K; Padamsee, 1983, IEEE Trans., Mag-19, 1322.
The thermal conductivity of Nb at these temperatures is strongly dependent on the purity of the metal. Dissolved gas impurities such as O, C, N and H go in between atomic sites (interstitially) and degrade the conductivity substantially. It is common to find 10-150 ppm (by weight) of interstitial impurities in commercially available pure Nb metal. In particular, since Nb has a very high affinity for O, this impurity is usually found at the highest levels. Other commonly found impurities such as Ta and W replace the regular atomic sites (substitutionally). In general substitutional impurities are far less harmful than interstitial impurities.
The electrical resistance of Nb at low temperature (e.g. 4.2K) in the normal state is also very sensitive to the total interstitial impurity content. This property correlates so well with impurity content that it is usually used as a measure of the total impurity content. The ratio of the electrical resistance of Nb at room temperature to the electrical resistance in the normal state at 4.2K is defined as the Residual Resistivity Ratio (RRR). Table 1 lists the RRR value for 1 weight ppm of the most commonly found imputities, Schulze, J. Metals, May 1981, p 33. Commercially pure Nb has typical RRR values between 20 and 40, corresponding to overall interstitial content of between 100 and 200ppm.
TABLE 1 ______________________________________ The Effect of Impurities on the RRR of Niobium Element RRR for 1 wt ppm ______________________________________ O 5000 N 3900 C 4100 H 1550 Ta 550,000 ______________________________________
From the theoretical calculation of Kadanoff and Martin, 1961, Phys. Rev., 124:670, which gives the ratio of thermal conductivity of the superconducting state to the normal state and from the Wiedemann Franz Law which connects normal state thermal conductivity with electrical conductivity, one can derive the rule: EQU RRR.apprxeq.400 K.sub.s (4.2K)
where K.sub.s is thermal conductivity of Niobium in the superconducting state at 4.2K measured in watts/cm-K. Commercially pure Nb has K.sub.s (4.2K) values between 0.05 and 0.1 watts/cm-K.
Reducing the interstitial impurity content has a profound effect on the thermal conductivity of Nb. To remove the dominant impurity, O, it is necessary to heat Nb at temperatures above 1900.degree. C. in a vacuum furnace of pressure lower than 10.sup.-8 torr, Fromm et al, 1969, Vacuum, 19:191. The pressure must be at these excellent levels in the hot zones. Unfortunately this is often found not to be the case for high temperature ultra high vacuum (UHV) furnaces of the type at High Energy Physics Laboratory, Stanford University, Stanford, California (HEPL), Brookhaven National Laboratory, Upton, N.Y. (BNL) or Kernforschungszentrum, Karlsruhe, West Germany (KFK). These furnaces are usually found to increase the O content. In other furnaces, e.g. at Max Planck Institute, Stuttgart, West Germany (MPI) where more efficient pumping arrangements exist, the ultra high vacuum, high temperature treatment to remove oxygen usually takes a long time (e.g. several hours for 3 mm Nb), because purification rates are controlled by evaporation rate of the oxides of Nb from the surface, and these are very slow: Schulze, Supra. Removal of carbon (decarburization) is accomplished by heating Nb between 1650.degree. and 1800.degree. C. in an oxygen atmosphere of 10.sup.-6 torr Fromm et al, Supra. This process usually increases the O content, so it becomes necessary to further apply difficult O removal methods described above. Removal of N takes place under conditions similar to O removal but more slowly: Cost et al, 1963, Acta Metallurgica, 11:231. Fortunately there is usually less N contamination in commercial Nb than O or C. Removal of H is the easiest. Heat treatment above 800.degree. C. in a moderately good vacuum (P&lt;10.sup.-5 torr) is effective in reducing H to below the 1 wt ppm level.
Solid state de-oxygenation is a process whereby a metal, which has a higher affinity to oxygen is brought into contact with the metal to be purified, and acts as a sink for interstitial diffusion of oxygen. Kirchheim et al, Acta Metallurgica, 1979, 27:869-878 and Scripta Metallurgica, 1977 11:651-654. The technique has been applied largely to vanadium using titanium and zirconium as sinks, Peterson et al, Metallurgical Transactions A, 1981, 12A:1127-1131: Yoshinari et al, J. Less Common Metals, 1981, 81:239-248, and to a lesser extent to niobium using zirconium as a sink, Shihata et al, Trans. Japan Institute of Metals, 1980, 21:639-644. The following reference appears material or related to the present invention:
Klatt et al Z. Fur. Metal, 1978, 67: 568-572, which describes the protection of niobium with a coating of titanium as a getter, in conjunction with molybdenum and silicon barriers.